Strainers have long been employed in filter or ion exchange resin environments in order to permit flow therethrough in either direction without a loss of the filtration or resin particles. Widely used among such strainers is a form of strainer known as the Johnson well screen which comprises a substantially cylindrical unit having in fact a polygonal cross section. The unit comprises a plurality of metal strips of substantially triangular cross section, the apex of said strip pointing inwardly to the center of the unit. The strips are held together by spacer units provided circumferentially around the inner circumference of the filter unit and parallel to the axis thereof. In such a unit the circumferential slots are open for approximately 90% of the circumference of the unit between any pair of slots. The triangular cross section of the strips permits a higher volume of through-put than would be possible if the space between the strips was uniformly small, that is to say restricted to the spacing necessary to exclude the particulate matter in the unit.
Filtration units and ion exchange columns require flow in two directions, an operating (generally a downward) direction and a backwash (generally upward) direction. In the operative, that is to say downward direction, screens of the Johnson well screen type are very satisfactory. Unfortunately problems occur in the use of such screens in the backwash operation.
The purpose of the backwash step in filters is to clean the filtration medium. In ion exchange resins, particularly in mixed bed (cation and anion) ion exchangers, the backwash step also serves, inter alia, to separate the anion and cation resins which have different densities and can therefore be separated by floatation in the column provided that an adequate degree of agitation is supplied. If the degree of agitation is not adequate the separation of the two types of resin is not complete leading to problems at subsequent points of the regeneration step which in turn do not permit the column to be returned to operation as rapidly as would be desirable, leading to increased losses in down time.
In the case of filtration media the problem is not quite so serious but leads to a loss of efficiency since the dirt collecting in the filtration medium is not adequately washed out. The reason for inadequate separation lies in the formation of dead spaces in the filtration or ion exchange tank due to the absence of adequate water velocity. These dead spaces typically are found on top of the strainer unit between strainers and in the zones proximate to the outer walls of the ion exchange or filter column. Heretofore the only available method of substantially reducing the dead space problem was by the rather close spacing of strainers used. Such a solution is expensive and raises additional design problems. It would therefore be desirable to provide a strainer which permits high volume, low velocity flow in the downward, or operative, direction while being able to provide high velocity efficient mixing flow in the backwash direction, still maintaining the capacity for separation of fine particles from the working liquid environment.